The image processing on the basis of the detector data conventionally also comprises the application of filters in addition to normalization. There are spatial filters, in which specific pixels are assigned values that are dependent on the values of the adjacent pixels, and likewise there is a temporal filtering process, in which the numerical values assigned to the individual pixels for a specific time are dependent on such values of the same pixel at times that are close to the specific time. Hence, in the spatial filtering process, there is a spatial neighborhood for each pixel, which neighborhood is taken into account during the filtering process, and there is a temporal neighborhood for any given time in the temporal filtering process.
An object of the filtering process is to suppress noise effects and to highlight the actual image signals more clearly. Image signals are identified by the relatively high local contrast. A standard deviation of the grayscale values can be calculated within a certain neighborhood of individual points. If the standard deviation is particularly large, this is the result of structures that are imaged in the X-ray image. In the case of small standard deviations, the assumption may be made that the effects are a result of noise. There is only little local averaging in the case of large standard deviations in order not to smudge the structures. In the case of small standard deviations, the numerical values are averaged in neighborhoods in order to reduce the noise effects. The process now is very sensitive to where the boundary is set between small and large standard deviations and hence where the boundary is set between strong and weak averaging.
It has been conventional for the filter functions in X-ray image recording devices to be fixedly prescribed for a particular model. Individual properties of the respective X-ray beam detectors are not taken into account. The functions can each be associated with individual modes of the X-ray image recording device, wherein the different modes for example can differ in the X-ray beam dose emitted by the X-ray beam source.
The noise behavior of X-ray beam detectors depends sensitively on individual parameters, for example on the thickness of a scintillator layer in the case of a flat-panel detector with precisely this layer. Depending on the quality of the manufacturing, there may moreover be variations in the electrical properties of the components; for example, if the utilized material varies in purity there is a different amount of noise. The internal gain factors provided by the components can also vary. However, these factors have a direct effect on the noise.